It's a known oUF issue. It's caused by the new anchoring system they implemented for the nameplates:

Code:

We added new restricted frames system that affects frame anchoring:
- Frames that are anchored to a restricted frame can only have their other
anchors set to frames within that same anchor hierarchy
- New restricted frames include
- Nameplates

I had this error pop up on nUI but not because of nameplates but due to nUI's game viewport anchoring to the UI frame. I've commented it out for now until I can figure out what its anchor family/hierarchy is to know what I have to do to fix it rofl.